1. Field of the Invention
The present invention relates to a tiltable-body apparatus with a tiltable body which can be reciprocally tilted about a twisting longitudinal axis, such as micro-sensors for sensing mechanical amounts, micro-actuators, and optical micro-scanners, and a method of fabricating the tiltable-body apparatus.
2. Description of the Related Background Art
It is well known that surface forces become more dominant than volume forces as the size of mechanical elements decreases and the influence of friction thus increases in such machines more than in normally-sized machines. Accordingly, in designing micro-machines, it is generally necessary to consider the reduction of the number of sliding portions and rotating portions as much as possible.
A conventional optical scanner with a tiltable body oscillating about a twisting longitudinal axis will be described. FIG. 1 illustrates the optical scanner disclosed in U.S. Pat. No. 4,317,611. FIG. 2 illustrates a disassembled view of this optical scanner to clearly show its internal structure. FIGS. 3 and 4 illustrate cross sections of a silicon thin plate 2020 taken along lines 2003 and 2006 in FIG. 1, respectively.
In the above optical scanner, a recess 2012 is formed in a substrate 2010 of an insulating material. A pair of driver electrodes 2014 and 2016 and a mirror support portion 2032 are provided on the bottom of the recess 2012. A pair of torsion bars 2022 and 2024 and a mirror 2030 are integrally formed in the silicon plate 2020. An upper surface of the mirror 2030 is coated with a highly-reflective material, and the mirror 2030 is rotatably supported by the torsion bars 2022 and 2024. The silicon plate 2020 is disposed above the substrate 2010 with a predetermined distance between the silicon plate 2020 and the driver electrodes 2014 and 2016 being set as illustrated in FIG. 3.
The silicon plate 2020 is electrically grounded. A voltage is alternately applied to each of the driver electrodes 2014 and 2016 to attract the mirror 2030 by an electrostatic force. The mirror 2030 is thus tilted about the longitudinal axis of the torsion bars 2022 and 2024.
The cross section of the torsion bars 2022 and 2024 has a shape of trapezoid as illustrated in FIG. 4. In a microstructure with such torsion bars, however, since the torsion bar is likely to bend in a direction perpendicular to its longitudinal axis, the microstructure can be easily affected by external vibrations and the longitudinal axis of the torsion bar can be easily shifted. Accordingly, it is difficult to attain an accurate driving in such a microstructure.
Therefore, when the above optical scanner is used in an optical scanning type display, its image and spot profile are likely to shift and vary due to the external vibrations. This disadvantage increases when the scanning type display is constructed in a small portable form.
The following structure has been proposed to solve the above-discussed disadvantage of the torsion bar. FIG. 5 illustrates a gimbal plate 2120 for a hard disc head disclosed in xe2x80x9c10th International Conference on Solid-State Sensors and Actuators (Transducers ""99) pp. 1002-1005xe2x80x9d. This gimbal plate 2120 is mounted on a tip portion of a suspension for the hard disc head so that rolling and pitching motions of a magnetic head are flexibly allowed. The gimbal plate includes a support frame 2131 which is rotatably supported by rolling torsion bars 2122 and 2124. There is also arranged inside the support frame 2131 a head support 2130 rotatably supported by pitching torsion bars 2126 and 2128. Twisting axes (indicated by dot-and-dash lines in FIG. 5) of rolling torsion bars 2122 and 2124 and pitching torsion bars 2126 and 2128 are orthogonal to each other, and hence, those torsion bars can achieve rolling and pitching motions of the head support 2130.
FIG. 6 is a cross-sectional view taken along a line 2106 of FIG. 5. As illustrated in FIG. 6, the cross section of each of the torsion bars 2122 and 2124 is T-shaped, and the gimbal plate 2120 has a structure with ribs.
A fabrication method of the above gimbal plate 2120 will be described with reference to FIGS. 7A to 7E. As illustrated in FIG. 7A, initially, a silicon wafer 2191 for molding is perpendicularly etched using an etching method such as ICP-RIE (Inductively Coupled Plasma-Reactive Ion Etching). The silicon wafer 2191 for molding can be re-used. A sacrificial layer 2192 of silicon oxide and phosphosilicate glass is then deposited on the silicon wafer 2191, as illustrated in FIG. 7B. After that, a poly-silicon layer 2193, which is to be the structure of the gimbal plate 2120, is formed as illustrated in FIG. 7C. The poly-silicon 2193 is then patterned as illustrated in FIG. 7D. Finally, the sacrificial layer 2192 is removed, and the poly-silicon layer 2193 is bonded to a patterned pad 2195 with an epoxy resin 2094, as illustrated in FIG. 7E.
The thus-fabricated torsion bar with the T-shaped cross section has the feature that its geometrical moment of inertia I is large while its polar moment of inertia J is relatively small, in contrast to a torsion bar having a circular or rectangular cross section. Therefore, the above torsion bar is relatively easy to twist while hard to bend. That is, this torsion bar has a sufficient compliance in a twisting direction and a high rigidity in a direction perpendicular to the twisting axis.
Further, in the above T-shaped torsion bar, the length for obtaining necessary compliance and permissible twisting angle is small, and hence, the torsion bar can be made compact in size.
Thus, a compact micro-gimbal plate with sufficient compliance in rolling and pitching directions and sufficient rigidity in other directions can be obtained.
However, the above-discussed microstructure has the following disadvantages.
1. In the torsion bar with the T-shaped cross section, a stress concentration is likely to occur at a portion 2150 of FIG. 6 when the torsion bar is twisted. Accordingly, the torsion bar is easy to break.
2. When the torsion bar with the T-shaped cross section is used, a twisting center of the torsion bar deviates from a center of gravity of the tiltable body. This phenomenon will be described with reference to FIGS. 8 and 9. FIG. 8 illustrates a T-shaped torsion bar 2922 one end of which is fixed and the other end of which supports a tiltable body 2930. FIG. 9 illustrates a side of the torsion bar 2922 viewed from a direction of view indicated by an arrow in FIG. 8. As illustrated by arrows in FIG. 9, since the twisting center of the T-shaped torsion bar 2922 deviates from the center of gravity of the tiltable body 2930, a vibratory force occurs in a direction perpendicular to the twisting longitudinal axis when the tiltable body 2930 is tilted. This causes unwanted noises in micro-sensors for mechanical amounts, unnecessary actions in micro-actuators, and deflection shifts of light in micro-optical scanners.
3. Internal loss of poly-silicon is larger than that of single crystal silicon. Accordingly, a mechanical Q-value of the poly-silicon is relatively small. The vibration amplitude cannot hence be increased when the tiltable body is driven by employing its mechanical resonance. Further, its energy efficiency is small since the driving loss is large.
It is an object of the present invention to provide a tiltable-body apparatus with good strength and performance including a tiltable body which can be reciprocally tilted about a twisting longitudinal axis, such as micro-sensors for sensing mechanical amounts, micro-actuators, and optical micro-scanners, and a method of fabricating the tiltable-body apparatus.
The present invention is generally directed to a tiltable-body apparatus including a frame member, a tiltable body, and a pair of torsion springs having a twisting longitudinal axis. The torsion springs are disposed along the twisting longitudinal axis opposingly with the tiltable body being interposed, support the tiltable body flexibly and rotatably about the twisting longitudinal axis relative to the frame member, and include a plurality of planar portions, compliant directions of which intersect each other when viewed along a direction of the twisting longitudinal axis. A center of gravity of the tiltable body is positioned on the twisting longitudinal axis of the torsion springs. This structure can provide a spring structure which can be readily twisted, but is hard to bend. Further, no unwanted vibratory force occurs in a direction perpendicular to the twisting longitudinal axis when the tiltable body is tilted.
More specifically, the following constructions can be preferably adopted based on the above fundamental construction.
The tiltable body can be a planar tiltable body, and at least one of the planar portions of the torsion springs extends slant to the planar tiltable body. Due to this structure, the torsion spring can be easily made difficult to bend in directions perpendicular to and parallel to the planar tiltable body.
The cross-sectional shape of each torsion spring perpendicular to the twisting longitudinal axis can be made 90-degree or 180-degree rotationally symmetric, and each torsion spring can be composed of a plurality of planar portions. This structure can provide a spring structure which can be further readily twisted, but is harder to bend.
Each torsion spring can be composed of a plurality of separate planar portions, longitudinal axes of which are set parallel to each other, and compliant directions of which intersect each other when viewed along the direction of the twisting longitudinal axis. Due to this structure, the separate planar portions reinforce each other such that the entire structure can have a high flexural rigidity and no vibratory forces perpendicular to the twisting longitudinal axis occurs at the tilting time. Further, since each planar portion has a simple cross section and separate, no great stress concentration occurs and the structure is drastically hard to break.
The cross-sectional shape of each torsion spring perpendicular to the twisting longitudinal axis can be made symmetric with respect to a plane including the twisting longitudinal axis. This structure also can provide a spring structure which can be further readily twisted, but is harder to bend.
The torsion springs can be formed of a single crystal material, such as single crystal silicon and quartz. In such a structure, its internal loss can be reduced, and a high energy efficiency can be attained. Further, a structure with a large mechanical Q-value can be achieved. The single crystal silicon is readily available, and excellent in mechanical characteristics (i.e., physical strength and durability are great, life is long, and specific gravity is small). When a (100) single crystal silicon is used, slant surfaces of the torsion springs can be readily achieved by (111) faces thereof.
Typically, the frame member, the tiltable body, and the torsion springs are integrally formed from a substrate of a single crystal material, such as single crystal silicon and quartz, by etching or the like.
The torsion springs can be formed by anisotropically etching the (100) single crystal silicon substrate, and slant surfaces of the torsion springs can be achieved by (111) faces of the single crystal silicon substrate. In this case, faces, relative to the (100) substrate face, of a root portion of each torsion spring, which connect to the silicon substrate, can be (111) faces of the single crystal silicon substrate. These torsion springs are hard to break since the (111) face is smoothly formed with high precision. Further, stress concentration to the root portion can be reduced, leading to an increase in reliability of the torsion springs.
The torsion springs can be formed by using a planar substrate, such as a silicon substrate, and performing a deep etching such as ICP-RIE. In this case, each torsion spring can be defined by faces perpendicular to the planar frame member and faces parallel to the planar frame member.
The cross section of each torsion spring perpendicular to the twisting longitudinal axis can have a shape of one of V, reversed-V, X, slash, broken-V, broken-reversed-V, crisscross, broken-crisscross, H, broken-H, N, and angular S.
Cross sections of the two torsion springs, which are opposingly arranged with the tiltable body being interposed, may be either the same, or different (see FIGS. 17A and 17B, for example).
Cross sections of the torsion springs perpendicular to the twisting longitudinal axis can be different and symmetric with each other with respect to a plane including the twisting longitudinal axis, or with respect to a plane including the twisting longitudinal axis and parallel to the planar tiltable body. In this structure, compliant directions of the torsion springs opposingly arranged with the tiltable body being interposed differ from each other, so that the spring structure can be readily twisted, but is hard to bend. Further, unnecessary modes of motion, and adverse influences of external disturbances due to the structure of one of the torsion springs can be offset by the structure of the other torsion spring.
Where each torsion spring includes a plurality of separate planar torsion bars, a cross section of each torsion spring may be symmetric with respect to a vertical line, or with respect to a horizontal line and a vertical line.
Angles of the torsion springs can be rounded by isotropic etching such that stress concentration on the angles of the torsion springs can be reduced.
The frame member can include an inner frame member and an outer frame member, and the tiltable body can include an inner tiltable body and an outer tiltable body which is the inner frame member for supporting the inner tiltable body through a pair of first torsion springs and is supported by the outer frame member through a pair of second torsion springs. In this structure, the inner tiltable body is supported flexibly and rotatably about a first twisting longitudinal axis of a pair of the first torsion springs, the outer tiltable body is supported flexibly and rotatably about a second twisting longitudinal axis of a pair of the second torsion springs, and pairs of the first and second torsion springs are disposed along the first and second twisting longitudinal axes opposingly with the inner and outer tiltable body being interposed, respectively. If necessary, more than two tiltable bodies can be flexibly and rotatably supported in such a manner (i.e., in a so-called gimbals fashion). Typically, the twisting longitudinal axes extend forming an angle of 90 degrees.
The tiltable-body apparatus can further include a detecting unit for detecting a relative displacement between the frame member and the tiltable body, and the apparatus can be constructed as a mechanical-amount sensor. The detecting unit detects a change in an electrostatic capacity between the frame member and the tiltable body through a change in a voltage therebetween, for example.
The tiltable-body apparatus can further include a driving unit for driving the tiltable body relative to the frame member, and the apparatus can be constructed as an actuator. The driving unit is typically composed of a stationary core formed of soft magnetic material, a coil wound on the stationary core, and a moving core bonded to the tiltable body. The moving core can be formed of either a soft magnetic material or a permanent magnet of hard magnetic material. When the moving core is formed of soft magnetic material, the driving principle is as follows. Magnetic poles of the soft magnetic material are not determined, and the soft magnetic material is attracted into a magnetic flux generated by the stationary core, such that a cross-sectional area where the soft magnetic material crosses the magnetic flux increases. The tiltable body is thus driven. Upon cease of the magnetic flux, the soft magnetic material is released from the magnetic flux.
When the moving core is formed of hard magnetic material, the driving principle is as follows. Magnetic poles of the hard magnetic material are determined, and the soft magnetic material is driven by an attractive force between different magnetic poles or a repulsive force between common magnetic poles. These two are electromagnetic actuators. Electrostatic forces can also be employed in an electrostatic actuator.
The tiltable-body apparatus can further include a driving unit for driving the tiltable body relative to the frame member, and a light deflecting unit for deflecting a beam of light impinging on the tiltable body, which is provided on the tiltable body, and the apparatus is constructed as an optical deflector. The driving unit can be constructed as described above. The light deflecting unit can be a light reflective mirror, or a diffraction grating. When the diffraction grating is used, a single beam can be deflected as a plurality of light beams (diffracted light).
The present invention is also directed to a scanning type display which includes the above-discussed optical deflector, a modulatable light source, and a control unit for controlling modulation of the modulatable light source and operation of the tiltable body of the optical deflector in an interlocking manner.
The present invention is further directed to a method of fabricating the above-discussed tiltable-body apparatus which includes the frame member formed of a (100) single crystal silicon substrate, the tiltable body formed of the (100) single crystal silicon substrate, and a pair of torsion springs having a twisting longitudinal axis, formed of the (100) single crystal silicon substrate, and including a plurality of planar portions defined by (100) and (111) faces of the single crystal silicon substrate. The method includes a step of depositing mask layers on both upper and lower surfaces of the (100) single crystal silicon substrate, respectively, a step of patterning the mask layers in accordance with configurations of the tiltable body and the torsion springs, and a step of anisotropically etching the (100) single crystal silicon substrate using the patterned mask layers. The anisotropic etching can be performed using an alkaline solution. The method may further include a step of rounding angles of the torsion springs by isotropic etching such that stress concentration on the angles of the torsion springs is reduced.
The present invention is further directed to a method of fabricating the above-discussed tiltable-body apparatus which includes a frame member formed of a planar substrate, a tiltable body formed of the planar substrate, and a pair of torsion springs having a twisting longitudinal axis, formed of the planar substrate, and including a plurality of planar portions defined by faces perpendicular to the planar substrate and faces parallel to the planar substrate. The method includes a step of depositing mask layers on both upper and lower surfaces of the planar substrate, respectively, a step of patterning the mask layers in accordance with configurations of the tiltable body and the torsion springs, a step of performing a deep etching of the planar substrate from one surface of the planar substrate, and a step of performing a deep etching of the planar substrate from the other surface of the planar substrate. The planar substrate can be a silicon substrate.
These advantages, as well as others will be more readily understood in connection with the following detailed description of the preferred embodiments of the invention in connection with the drawings.